316 REPORT OF STATE BOARD OF HORTICULTURE. 



THE APPLE-PLANT LOUSE. 



{Aphis mali Koch). 



Bv John B. Smith. .SC.D. 



For several years }3ast an increasing number of complaints has been 

 received of injury caused by plant lice on apple trees, young stock suffering- 

 most severely, but older, bearing- trees being by no means exempt. The 

 insects appear with the foliage, and where they are at all numerous the 

 leaves begin to curl, and growth is checked early in summer. The aphids 

 excrete a sweet, sticky liquid, called ''honey dew," in great abundance, and 

 on this a black soot fungus develops, which chokes the leaves, causing them 

 to become dry, turn brown, and sometimes to drop. Often the tips of the 

 shoots are killed, though more frequently they are stunted, and the young 

 tree makes no satisfactory growth, barely maintaining itself in many cases. 

 r)n bearing trees the young fruit is checked in development, becomes sooty, 

 crippled, and never ripens properly. 



The injury is often severe, and seems to be steadily increasing. Ten years 

 ago the insects were i^arely seen in large numbers ; in 1898 and 1899 the dam- 

 age caused by them in some orchards exceeded that of all other insects com- 

 bined. 



Not all varieties of apples are equally susceptible to attack ; but as no 

 notes were made until so late as to make them incomplete, no definite infor- 

 mation on this point can be given at this time. 



In the published accounts of the apple louse, it is said that the eggs are 

 on the trees during the winter, hidden in crevices, laid at the base of the 

 buds or wherever else they can be suitably placed. From these eggs come 

 little green lice or aphids in early spring, and these develop into what are 

 known as "stem-mothers" — wingless and sexless forms that produce living 

 young. 



The young from these "stem- mothers"' become winged in due time, and 

 migrate to wheat or other grasses, where they propagate during the summer. 

 In fall another set of winged forms — "return migrants" — are developed, and 

 from these come the sexed individuals. They copulate and the female lays 

 eggs, thus completing the life-history. 



There is no doubt that there is a species that has such a life-cycle, but 

 there is another that has been confused with it, and a careful reading of the 

 various accounts shows that observers have not discriminated between the 

 two— as when it was suggested that both winged and wingless males occur 

 in the seme species. 



